The Annual Modulation Signature for Dark Matter: DAMA/LIBRA ...

Hindawi Publishing Corporation Advances in High Energy Physics Volume 2014, Article ID 605659, 10 pages http://dx.doi.org/10.1155/2014/605659

Research Article The Annual Modulation Signature for Dark Matter: DAMA/LIBRA-Phase1 Results and Perspectives Rita Bernabei,1,2 Pierluigi Belli,2 Fabio Cappella,3,4 Vincenzo Caracciolo,5 Simone Castellano,5 Riccardo Cerulli,5 Chang Jang Dai,6 Annelisa d’Angelo,3,4 Silio d’Angelo,1,2 Alessandro Di Marco,1,2 H. L. He,6 Antonella Incicchitti,4 H. H. Kuang,6 X. H. Ma,6 Francesco Montecchia,2,7 X. D. Sheng,6 Rui Guang Wang,6 and Zi-Piao Ye6,8 1

Dipartimento di Fisica, Università di Roma “Tor Vergata”, 00133 Rome, Italy INFN, sez. Roma “Tor Vergata”, 00133 Rome, Italy 3 Dipartimento di Fisica, Università di Roma “La Sapienza”, 00185 Rome, Italy 4 INFN, sez. Roma, 00185 Rome, Italy 5 Laboratori Nazionali del Gran Sasso, INFN, 67100 Assergi, Italy 6 Institute of High Energy Physics, Chinese Academy of Sciences, P.O. Box 918-3, Beijing 100049, China 7 Dipartimento di Ingegneria Civile e Ingegneria Informatica, Università di Roma “Tor Vergata”, 00133 Rome, Italy 8 Maths & Physics College, Jinggangshan University, Jián 343009, China 2

Correspondence should be addressed to Pierluigi Belli; [email protected] Received 13 January 2014; Accepted 19 March 2014; Published 24 June 2014 Academic Editor: Anselmo Meregaglia Copyright © 2014 Rita Bernabei et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The publication of this article was funded by SCOAP3 . The results obtained with the total exposure of 1.04 ton × yr collected by DAMA/LIBRA-phase1 deep underground at the Gran Sasso National Laboratory (LNGS) of the I.N.F.N. during 7 annual cycles are summarized. The DAMA/LIBRA-phase1 and the former DAMA/NaI data (cumulative exposure 1.33 ton × yr, corresponding to 14 annual cycles) give evidence at 9.3 𝜎 C.L. for the presence of Dark Matter (DM) particles in the galactic halo, on the basis of the exploited model independent DM annual modulation signature by using highly radiopure NaI(Tl) target. The modulation amplitude of the single-hit events in the (2–6) keV energy interval is (0.0112 ± 0.0012) cpd/kg/keV; the measured phase is (144 ± 7) days and the measured period is (0.998 ± 0.002) yr; values are in a good well in agreement with those expected for DM particles. No systematic or side reactions able to mimic the exploited DM signature have been found or suggested by anyone over more than a decade. Some of the perspectives of the presently running DAMA/LIBRA-phase2 are outlined.

1. Introduction The presently running DAMA/LIBRA [1–11] experiment, as the former DAMA/NaI [12–41], has the main aim to investigate the presence of DM particles in the galactic halo by exploiting the model independent DM annual modulation signature (originally suggested in [42, 43]). Moreover, the developed highly radiopure NaI(Tl) target-detectors [1] assure as well sensitivity to a wide range of DM candidates, interaction types, and astrophysical scenarios.

As a consequence of the Earth’s revolution around the Sun, the Earth should be crossed by a larger flux of DM particles around ≃2 June and by a smaller one around ≃2 December. This DM annual modulation signature is very distinctive since the effect induced by DM particles must simultaneously satisfy all the following requirements: (1) the rate must contain a component modulated according to a cosine function (2) with one year period and (3) a phase that peaks roughly ≃2 June; (4) this modulation must only be found in a well-defined low energy range, where DM particle

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2. The Results of DAMA/LIBRA-Phase1 and DAMA/NaI The total exposure of DAMA/LIBRA-phase1 is 1.04 ton × yr in seven annual cycles; when including also that of the first generation DAMA/NaI experiment, it is 1.33 ton × yr,

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induced events can be present; (5) it must apply only to those events in which just one detector of many actually “fires” (single-hit events), since the DM particle multi-interaction probability is negligible; (6) the modulation amplitude in the region of maximal sensitivity must be ≃7% for usually adopted halo distributions (see, e.g., [8, 23, 24]), but it can be larger in case of some possible scenarios such as, for example, those in [44–48] (even up to ≃30%). Thus, this signature is model-independent, is very effective, and, in addition, it allows the test of a large range of cross sections and of halo densities. This DM signature might be mimicked only by systematic effects or side reactions able to account for the whole observed modulation amplitude and to simultaneously satisfy all the requirements given above. No one is available [1– 3, 7, 8, 22–24]. The full description of the DAMA/LIBRA setup during phase1 and other related arguments have been discussed in detail in [1–4, 8] and references therein. Here, we just remind the reader that the sensitive part of this setup is made of 25 highly radiopure NaI(Tl) crystal scintillators (5 rows by 5-column matrix) having 9.70 kg mass each one. In each detector two 10 cm long UV light guides (made of Suprasil B quartz) act also as optical windows on the two end faces of the crystal and are coupled with two low background photomultipliers (PMTs) working in coincidence at single photoelectron level. The low background 9265-B53/FL and 9302-A/FL PMTs, developed by EMI-Electron Tubes with dedicated R&Ds, were used in the phase1; for details, see [1, 21, 23] and references therein. The detectors are housed in a sealed low-radioactive copper box installed in the center of a low-radioactive Cu/Pb/Cd-foils/polyethylene/paraffin shield; moreover, about 1 m concrete (made from the Gran Sasso rock material) almost fully surrounds (mostly outside the barrack) this passive shield, acting as a further neutron moderator. A threefold-level sealing system prevents the detectors from being in contact with the environmental air of the underground laboratory [1]. The light response of the detectors during phase1 typically ranges from 5.5 to 7.5 photoelectrons/keV, depending on the detector. The hardware threshold of each PMT is at single photoelectron, while a software energy threshold of 2 keV electron equivalent (hereafter keV) is used [1, 21]. Energy calibration with Xrays/𝛾 sources are regularly carried out in the same running condition down to few keV [1]; in particular, double coincidences due to internal X-rays from 40 K (which is at ppt levels in the crystals) provide (when summing the data over long periods) a calibration point at 3.2 keV close to the software energy threshold (for details, see [1]). The radiopurity, the procedures, and details are discussed in [1– 4, 8] and references therein.

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Figure 1: Experimental residual rate of the single-hit scintillation events measured by DAMA/LIBRA-phase1 in the (2–6) keV energy interval as a function of the time. The data points present the experimental errors as vertical bars and the associated time bin width as horizontal bars. The superimposed curves are the cosinusoidal functions behaviour A cos 𝜔(𝑡 − 𝑡0 ) with a period 𝑇 = 2𝜋/𝜔 = 1 yr, a phase 𝑡0 = 152.5 day (June 2nd) and modulation amplitudes, 𝐴, equal to the central values obtained by best fit on the data points of the entire DAMA/LIBRA-phase1. The dashed vertical lines correspond to the maximum expected for the DM signal (June 2nd), while the dotted vertical lines correspond to the minimum.

corresponding to 14 annual cycles. The variance of the cosine (𝛼 − 𝛽2 ) = (⟨cos2 ⟩ − ⟨cos⟩2 ), where the averages ⟨⋅ ⋅ ⋅ ⟩ are taken over the periods in which the data taking was on, is 0.518 during the DAMA/LIBRA-phase1, showing that the setup has been operational evenly throughout the years (the expectation value for full-year data taking is (𝛼 − 𝛽2 ) = 0.5). For details, see [2–4, 8]. The total number of events collected for the routine calibrations during the entire DAMA/LIBRA-phase1 is about 9.6 × 107 , while about 3.5 × 106 events/keV have been collected for the evaluation of the acceptance window efficiency for noise rejection near energy threshold [1]. The duty cycle of the experiment is high [4]; the routine calibrations and, in particular, those related to the acceptance windows efficiency mainly affect it. Figure 1 shows the time behaviour of the experimental residual rates of the single-hit scintillation events in the (2–6) keV energy interval for DAMA/LIBRA-phase1. The residuals of the DAMA/NaI data (0.29 ton × yr) are given in [2, 8, 23, 24]. We remind the reader that these residual rates are calculated from the measured rate of the single-hit events after subtracting the constant part: ⟨𝑟𝑖𝑗𝑘 − flat𝑗𝑘 ⟩𝑗𝑘 . Here, 𝑟𝑖𝑗𝑘 is the rate in the considered 𝑖th time interval for the 𝑗th detector in the 𝑘th energy bin, while flat𝑗𝑘 is the rate of the 𝑗th detector in the 𝑘th energy bin averaged over the cycles; it is of order of ≲1 cpd/kg/keV [1, 2, 49]. The average is made on all the detectors (𝑗 index) and on all the energy bins (𝑘 index) which constitute the considered energy interval. The weighted mean of the residuals must obviously be zero over one cycle. The 𝜒2 test excludes the hypothesis of absence of modulation in the data: 𝜒2 /d.o.f. = 83.1/50 and the 𝑃 value is 𝑃 = 2.2 × 10−3 for the (2–6) keV energy interval. When fitting

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the single-hit residual rate of DAMA/LIBRA-phase1 (Figure 1) together with the DAMA/NaI ones, with the function, A cos 𝜔(𝑡 − 𝑡0 ), considering a period 𝑇 = 2𝜋/𝜔 = 1 yr and a phase 𝑡0 = 152.5 day (June 2nd) as expected by the DM annual modulation signature, the following modulation amplitude is obtained: 𝐴 = (0.0110 ± 0.0012) cpd/kg/keV corresponding to 9.2𝜎 C.L. (𝜒2 of the fit is 70.4 over 86 d.o.f.). When the period and the phase are kept free in the fitting procedure, the modulation amplitude is (0.0112 ± 0.0012) cpd/kg/keV (9.3𝜎 C.L.), the period 𝑇 = (0.998 ± 0.002) year, and the phase 𝑡0 = (144 ± 7) day. The period and the phase are well compatible with expectations for a DM annual modulation signal. In particular, the phase is consistent with about June 2nd and is fully consistent with the value independently determined by Maximum Likelihood analysis (see later). For completeness, we recall that a slight energy dependence of the phase could be expected in case of possible contributions of nonthermalized DM components to the galactic halo, such as, for example, the SagDEG stream [26, 50–54] and the caustics [55]. For more details, see [4]. The modulation amplitudes singularly calculated for each annual cycle of DAMA/NaI and DAMA/LIBRA-phase1 are compatible among them and are normally fluctuating around their best fit values [2–4]. In particular, for the (2–6) keV energy interval, the 𝜒2 is 10.8 over 13 d.o.f. corresponding to an upper tail probability of 63%, while the run test yields a lower tail probability of 23%. This analysis confirms that the data collected in all the annual cycles with DAMA/NaI and DAMA/LIBRA-phase1 are statistically compatible and can be considered together, on the contrary of the statements in [56]. The DAMA/LIBRA-phase1 single-hit residuals of Figure 1 and those of DAMA/NaI have also been investigated by a Fourier analysis. The data analysis procedure has been described in detail in [8]. A clear peak corresponding to a period of 1 year (see Figure 2) is evident for the (2–6) keV energy interval; the same analysis in the (6–14) keV energy region shows instead only aliasing peaks. No other structure at different frequencies has been observed (see also [8]). Absence of any other significant background modulation in the energy spectrum has been verified in energy regions not of interest for DM. (In fact, the background in the lowest energy region is essentially due to “Compton” electrons, Xrays, and/or Auger electrons, muon induced events, etc., which are strictly correlated with the events in the higher energy region of the spectrum. Thus, if a modulation detected in the lowest energy region was due to a modulation of the background (rather than to a signal), an equal or larger modulation in the higher energy regions should be present.) For example, the measured rate integrated above 90 keV, 𝑅90 , as a function of the time has been analysed [4]. Similar result is obtained when comparing the single-hit residuals in the (2–6) keV with those in other energy intervals; for example, Figure 3 shows the single-hit residuals in the (2–6) keV and in the (6–14) keV energy regions for the entire DAMA/LIBRAphase1 data as if they were collected in a single annual cycle (i.e., binning in the variable time from the Jan 1st of each annual cycle). It is worth noting that the obtained results account for whatever kind of background and, in

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Figure 2: Power spectrum of the measured single-hit residuals in the (2–6) keV (solid lines) and (6–14) keV (dotted lines) energy intervals calculated according to [8], including also—as usual in DAMA analyses—the treatment of the experimental errors and of the time binning. The data refer to DAMA/NaI and DAMA/LIBRAphase1. As it can be seen, the principal mode present in the (2– 6) keV energy interval corresponds to a frequency of 2.737×10−3 d−1 (vertical lines), corresponding to a period of ≃1 year. A similar peak is not present in the (6–14) keV energy interval.

addition, no background process able to mimic the DM annual modulation signature (that is able to simultaneously satisfy all the peculiarities of the signature and to account for the measured modulation amplitude) is available (see also discussions, for example, in [1–4, 7, 8, 49, 57–62]). A further relevant investigation in the DAMA/LIBRAphase1 data has been performed by applying the same hardware and software procedures, used to acquire and to analyse the single-hit residual rate, to the multiple-hit one. (A multiple-hit event is defined when more scintillating pulses arrive within a time window of about 600 ns.) In fact, since the probability that a DM particle interacts in more than one detector is negligible, a DM signal can be present just in the single-hit residual rate. Thus, the comparison of the results of the single-hit events with those of the multiple-hit ones corresponds practically to compare between them the cases of DM particles beam-on and beam-off. This procedure also allows an additional test of the background behaviour in the same energy interval where the positive effect is observed. In particular, in Figure 4, the residual rates of the singlehit events measured over the DAMA/LIBRA-phase1 annual cycles are reported, as collected in a single cycle, together with the residual rates of the multiple-hit events, in the (2– 6) keV energy interval. While, as already observed, a clear modulation, satisfying all the peculiarities of the DM annual modulation signature, is present in the single-hit events, the fitted modulation amplitude for the multiple-hit residual rate is well compatible with zero: −(0.0005 ± 0.0004) cpd/kg/keV

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Figure 3: Experimental single-hit residuals in the (2–6) keV and in the (6–14) keV energy regions for the entire DAMA/LIBRA-phase1 data as if they were collected in a single annual cycle (i.e., binning in the variable time from the Jan 1st of each annual cycle). The data points present the experimental errors as vertical bars and the associated time bin width as horizontal bars. The initial time of the figures is taken at August 7th. A clear modulation satisfying all the peculiarities of the DM annual modulation signature is present in the lowest energy interval with 𝐴 = (0.0088 ± 0.0013) cpd/kg/keV, while it is absent just above 𝐴 = (0.00032 ± 0.00076) cpd/kg/keV.

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Figure 4: Experimental residual rates of DAMA/LIBRA-phase1 single-hit events (open circles), class of events to which DM events belong, and, for multiple-hit events (filled triangles), class of events to which DM events do not belong. They have been obtained by considering for each class of events the data as collected in a single annual cycle and by using in both cases the same identical hardware and the same identical software procedures. The initial time of the figure is taken on August 7th. The experimental points present the errors as vertical bars and the associated time bin width as horizontal bars. Analogous results were obtained for the DAMA/NaI data [24].

in the energy region (2–6) keV. Thus, again evidence of annual modulation with proper features as required by the DM annual modulation signature is present in the single-hit residuals (events class to which the DM particle induced events belong), while it is absent in the multiple-hit residual rate (event class to which only background events belong). Similar results were also obtained for the last two annual cycles of the DAMA/NaI experiment [24]. Since the same identical hardware and the same identical software procedures have been used to analyse the two classes of events, the obtained

result offers an additional strong support for the presence of a DM particle component in the galactic halo. The annual modulation present at low energy can also be pointed out by depicting—as a function of the energy— the modulation amplitude, 𝑆𝑚,𝑘 , obtained by maximum likelihood method over the data considering 𝑇 = 1 yr and 𝑡0 = 152.5 day. For such purpose, the likelihood function of the single-hit experimental data in the 𝑘th energy bin is defined 𝑁 as Lk = Π𝑖𝑗 𝑒−𝜇𝑖𝑗𝑘 (𝜇𝑖𝑗𝑘𝑖𝑗𝑘 /𝑁𝑖𝑗𝑘 !), where 𝑁𝑖𝑗𝑘 is the number of events collected in the 𝑖th time interval (hereafter 1 day), by the 𝑗th detector and in the 𝑘th energy bin. 𝑁𝑖𝑗𝑘 follows a Poisson’s distribution with expectation value 𝜇𝑖𝑗𝑘 = [𝑏𝑗𝑘 + 𝑆𝑖𝑘 ]𝑀𝑗 Δ𝑡𝑖 Δ𝐸𝜖𝑗𝑘 . The 𝑏𝑗𝑘 are the background contributions, 𝑀𝑗 is the mass of the 𝑗th detector, Δ𝑡𝑖 is the detector running time during the 𝑖th time interval, Δ𝐸 is the chosen energy bin, 𝜖𝑗𝑘 is the overall efficiency (for details, see, e.g., [1]). Moreover, the signal can be written as 𝑆𝑖𝑘 = 𝑆0,𝑘 + 𝑆𝑚,𝑘 ⋅ cos 𝜔(𝑡𝑖 − 𝑡0 ), where 𝑆0,𝑘 is the constant part of the signal and 𝑆𝑚,𝑘 is the modulation amplitude. The usual procedure is to minimize the function 𝑦𝑘 = −2 ln(Lk ) − const for each energy bin; the free parameters of the fit are the (𝑏𝑗𝑘 + 𝑆0,𝑘 ) contributions and the 𝑆𝑚,𝑘 parameter. Hereafter, the index 𝑘 is omitted for simplicity. In Figure 5, the obtained 𝑆𝑚 are shown in each considered energy bin (there Δ𝐸 = 0.5 keV) when the data of DAMA/NaI and DAMA/LIBRA-phase1 are considered. It can be inferred that positive signal is present in the (2–6) keV energy interval, while 𝑆𝑚 values compatible with zero are present just above. In fact, the 𝑆𝑚 values in the (6–20) keV energy interval have random fluctuations around zero with 𝜒2 equal to 35.8 for 28


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Table 1: Best fit values for the (2–6) and (6–14) keV energy intervals (1𝜎 errors) for 𝑆𝑚 versus 𝑍𝑚 and 𝑌𝑚 versus 𝑡∗ , considering the cumulative exposure of DAMA/NaI and DAMA/LIBRA-phase1. See also Figure 6.

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𝑆𝑚 (cpd/kg/keV) (0.0106 ± 0.0012) (0.0001 ± 0.0007)

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Figure 5: 𝑆𝑚 variable as a function of the energy for the total cumulative exposure 1.33 ton × yr. The energy bin is 0.5 keV. A clear modulation is present in the lowest energy region, while 𝑆𝑚 values compatible with zero are present just above. In fact, the 𝑆𝑚 values in the (6–20) keV energy interval have random fluctuations around zero with 𝜒2 equal to 35.8 for 28 degrees of freedom (upper tail probability of 15%).

degrees of freedom (upper tail probability of 15%). All this confirms the previous analyses. As described in [2–4, 8], the observed annual modulation effect is well distributed in all the 25 detectors at 95% C.L. Among further additional tests, the analysis of the modulation amplitudes as a function of the energy separately for the nine inner detectors and the remaining external ones has been carried out for the entire DAMA/LIBRA-phase1. The obtained values are fully in agreement; in fact, the hypothesis that the two sets of modulation amplitudes as a function of the energy belong to the same distribution has been verified by 𝜒2 test, obtaining 𝜒2 /d.o.f. = 3.9/4 and 8.9/8 for the energy intervals (2–4) and (2–6) keV, respectively (Δ𝐸 = 0.5 keV). This shows that the effect is also well shared between inner and outer detectors. Let us, finally, release the assumption of a phase 𝑡0 = 152.5 day in the procedure to evaluate the modulation amplitudes. In this case, the signal can be written as 𝑆𝑖𝑘 = 𝑆0,𝑘 + 𝑆𝑚,𝑘 cos 𝜔 (𝑡𝑖 − 𝑡0 ) + 𝑍𝑚,𝑘 sin 𝜔 (𝑡𝑖 − 𝑡0 ) = 𝑆0,𝑘 + 𝑌𝑚,𝑘 cos 𝜔 (𝑡𝑖 − 𝑡∗ ) .

(1)

For signals induced by DM particles, one should expect the following: (i) 𝑍𝑚,𝑘 ∼ 0 (because of the orthogonality between the cosine and the sine functions); (ii) 𝑆𝑚,𝑘 ≃ 𝑌𝑚,𝑘 ; (iii) 𝑡∗ ≃ 𝑡0 = 152.5 day. In fact, these conditions hold for most of the dark halo models; however, as mentioned above, slight differences can be expected in case of possible contributions from nonthermalized DM components, such as, for example, the SagDEG stream [26, 50–54] and the caustics [55]. Considering cumulatively the data of DAMA/NaI and DAMA/LIBRA-phase1 (exposure 1.33 ton × yr) the obtained

𝑌𝑚 (cpd/kg/keV) (0.0107 ± 0.0012) (0.0001 ± 0.0008)

𝑡∗ (day) (149.5 ± 7.0) Undefined

2𝜎 contours in the plane (𝑆𝑚 , 𝑍𝑚 ) for the (2–6) keV and (6– 14) keV energy intervals are shown in Figure 6(a), while in Figure 6(b) the obtained 2𝜎 contours in the plane (𝑌𝑚 , 𝑡∗ ) are depicted. The best fit values for the (2–6) and (6–14) keV energy intervals (1𝜎 errors) for 𝑆𝑚 versus 𝑍𝑚 and 𝑌𝑚 versus 𝑡∗ are reported in Table 1. Finally, setting 𝑆𝑚 in (1) to zero, the 𝑍𝑚 values as function of the energy have also been determined by using the same procedure. The values of 𝑍𝑚 as a function of the energy is reported in Figure 7; they are expected to be zero. The 𝜒2 test applied to the data supports the hypothesis that the 𝑍𝑚 values are simply fluctuating around zero; in fact, for example, in the (2–14) keV and (2–20) keV energy region the 𝜒2 /d.o.f. are equal to 23.0/24 and 46.5/36 (probability of 52% and 11%), respectively. The behaviour of the phase 𝑡∗ variable as function of energy is shown in Figure 8 for the cumulative exposure of DAMA/NaI and DAMA/LIBRA-phase1 (1.33 ton × yr). No modulation is present above 6 keV and the phase is undetermined. Sometimes naive statements were put forward as the fact that, in nature, several phenomena may show some kind of periodicity. It is worth noting that the point is whether they might mimic the annual modulation signature in DAMA/LIBRA (and former DAMA/NaI), that is, whether they might be not only quantitatively able to account for the observed modulation amplitude but also able to contemporaneously satisfy all the requirements of the DM annual modulation signature. The same is also for side reactions. This has already been deeply investigated in [1–4] and references therein; the arguments and the quantitative conclusions, presented there, also apply to the entire DAMA/LIBRAphase1 data. Additional arguments can be found in [7, 8, 49, 57–62]. Firstly, in order to continuously monitor the running conditions, several pieces of information are acquired with the production data and quantitatively analysed. In particular, all the time behaviour of the running parameters, acquired with the production data, have been investigated. Table 2 shows the modulation amplitudes obtained for each annual cycle when fitting the time behaviour of the values of the main parameters including a cosine modulation with the same phase and period as for DM particles. As can be seen, all the measured amplitudes are well compatible with zero. No modulation has been found in any possible source of systematics or side reactions; thus, cautious upper limits (90% C.L.) on possible contributions to the DAMA/LIBRA measured modulation amplitude are summarized in Table 3 (also see [2–4]). It is worth noting that they do not quantitatively account for the measured modulation amplitudes and also are

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Figure 6: 2𝜎 contours in the plane (𝑆𝑚 , 𝑍𝑚 ) (a) and in the plane (𝑌𝑚 , 𝑡 ) (b) for the (2–6) keV and (6–14) keV energy intervals. The contours have been obtained by the maximum likelihood method, considering the cumulative exposure of DAMA/NaI and DAMA/LIBRA-phase1. A modulation amplitude is present in the lower energy intervals and the phase agrees with that expected for DM induced signals. See text.

Table 2: Modulation amplitudes (1𝜎 error) obtained by fitting the time behaviours of the main running parameters including a possible annual modulation with phase and period as for DM particles. These running parameters, acquired with the production data are (i) the operating temperature of the detectors; (ii) the HP Nitrogen flux in the inner Cu box housing the detectors; (iii) the pressure of the HP Nitrogen atmosphere of the inner Cu box housing the detectors; (iv) the environmental Radon in the inner part of the barrack from which the detectors are, however, excluded; (v) the hardware rate above single photoelectron threshold. All the measured amplitudes are compatible with zero. Temperature (∘ C) Flux (L/h) Pressure (10−3 mbar) Radon (Bq/m3 ) Hardware rate (10−2 Hz) Temperature (∘ C) Flux (L/h) Pressure (10−3 mbar) Radon (Bq/m3 ) Hardware rate (10−2 Hz)

DAMA/LIBRA-1 −(0.0001 ± 0.0061) (0.13 ± 0.22) (15 ± 30) − (0.029 ± 0.029) − (0.20 ± 0.18) DAMA/LIBRA-5 (0.0001 ± 0.0036) − (0.01 ± 0.21) − (0.8 ± 1.2) (0.021 ± 0.037) (0.03 ± 0.14)

DAMA/LIBRA-2 (0.0026 ± 0.0086) (0.10 ± 0.25) − (13 ± 25) − (0.030 ± 0.027) (0.09 ± 0.17) DAMA/LIBRA-6 (0.0007 ± 0.0059) − (0.01 ± 0.15) (0.7 ± 1.3) − (0.028 ± 0.036) (0.08 ± 0.11)

not able to simultaneously satisfy all the many requirements of the signature. Similar analyses have also been done for the seven annual cycles of DAMA/NaI [23, 24]. In conclusion, the model-independent DAMA results give evidence (at 9.3𝜎 C.L. over 14 independent annual cycles) for the presence of DM particles in the galactic halo. In order to perform corollary investigation on the nature of the DM particles, model-dependent analyses are necessary; thus, many theoretical and experimental parameters and models are possible and many hypotheses must also be exploited.

DAMA/LIBRA-3 (0.001 ± 0.015) − (0.07 ± 0.18) (22 ± 27) (0.015 ± 0.029) − (0.03 ± 0.20) DAMA/LIBRA-7 (0.0000 ± 0.0054) − (0.00 ± 0.14) − (2.6 ± 5.5) (0.012 ± 0.047) (0.06 ± 0.10)

DAMA/LIBRA-4 (0.0004 ± 0.0047) − (0.05 ± 0.24) (1.8 ± 7.4) − (0.052 ± 0.039) (0.15 ± 0.15)

In particular, the obtained DAMA model independent evidence is compatible with a wide set of scenarios regarding the nature of the DM candidate and related astrophysical, nuclear, and particle Physics. For example, some given scenarios and parameters are discussed in [2, 8, 13–20, 22, 23, 26]. Further large literature is available on the topics (see e.g., in [8]). Moreover, both the negative results and all the possible positive hints, achieved so far in the field, are largely compatible with the DAMA model-independent DM annual modulation results in many scenarios considering also the existing experimental and theoretical uncertainties; the same


7

Table 3: Summary of the results obtained by investigating possible sources of systematics or side processes [1–4, 7, 8, 49, 57–62]. None able to give a modulation amplitude different from zero has been found; thus, cautious upper limits (90% C.L.) on the possible contributions to the measured modulation amplitude have been calculated and are shown here for DAMA/LIBRA-phase1 as done before for the seven annual cycles of DAMA/NaI [23, 24]. Source Radon Temperature Noise Energy scale Efficiencies Background

Zm (cpd/kg/keV)

Side reactions

Main comment

Cautious upper limit (90% C.L.)

Sealed Cu Box in HP Nitrogen atmosphere, 3 levels of sealing Air conditioning + huge heat capacity Efficient rejection Routine + intrinsic calibrations Regularly measured No modulation above 6 keV; no modulation in the (2–6) keV multiple-hit events; this limit includes all possible sources of background From muon flux variation measured by MACRO In addition: no effect can mimic the signature